Olds says the study looked at the association between sleep and factors such as age, sex and household income, type of day, and weight.

It included 4033 children aged nine to 18 years, who reported the time they fell asleep and woke over a combined total of 9053 individual nights.

Days were categorised into four types: bed on a non-school day, wake on a school day (NS-S); bed on a school day, wake on a school day (S-S); bed on school day, wake on a non-school day (S-NS); and bed on a non-school day, wake on a non-school day (NS-NS).

He says the participants reported their activities in time slices as fine as five minutes.

Olds says the study found that adolescents sleep least on the Sunday to Monday night (NS-S) at 553 minutes, and most on non-school-non-school nights (Saturdays and holidays) at 603 minutes.

Starting on empty

The survey found that older children woke up 5 to 6 minutes later than average on Monday and were already in deficit with the time they needed to sleep.

"We go to bed late on a Friday and Saturday and don't want to leave the weekend behind [by going to bed late on a Sunday]," Olds says.

He says this means adolescents are starting the week in sleep deficit.

"We suspect it has bad consequences," he says pointing to studies that show sleep deprivation is associated with impaired immune function and psychological problems such as memory and attention deficits.

Olds says the "norm" for sleep time decreases as children grow up.

On average, nine-year-olds sleep between 10 and 10.5 hours per night. This decreases by about nine minutes per night with each year of age, until they reach the age of 18 years.

But Olds says the decrease in sleep time was greater on school-school (S-S) and non-school to school (NS-S) nights averaging 15-16 minutes per night per year.

He says this suggests the older teenagers are staying up later, but still having to wake at the same time for school.

They don't make up for the sleep lost until Saturday night.

"There is a lot of evidence that children are sleep deprived during the school week," he says.

However he says a Korean study showed the difference between school and non-school nights was extreme.

During the school week, schooling pressures meant an average Korean 18-year-old slept a mere 4.5 hours' night.

However on non-school nights they slept more than half the day averaging 13.5 hours' sleep.

Parental guidelines

The study also found that children from higher socio-economic households sleep more; a trend Olds says may be connected to a household's ability to enforce rules and regulations.

He says as part of the study they revisited a number of schools that had participated in a 1985 study and asked the same aged children to report their sleep times.

They found on average Australian children are sleeping 30 minutes less than their peers did in 1985.

"It's entirely down to a later bed time," says Olds.

He says the findings should be used to issue guidelines to parents on recommended sleep times.